With regard to the action of a plasticizer in the membrane composition of a polymeric ion carrier membrane in a liquid membrane-type electrode of an ion sensor or the like in which the polymeric membrane is substituted for a glass membrane, W. Simon et al have found that the dielectric constant of the membrane solution (plasticizer) (a water-immiscible liquid of low vapor pressure, compatible with PVC, no functional group of which can undergo protonation reaction) enhances the selectivity of a monovalent cation of the same size ["Ion Enzyme Electrodes in Biology & Medicine", W. Simon, Int. Workshop, pp. 22-37 (1976)]. As a result, o-NPOE [o-nitrophenyl-n-octylether 24 (20.degree. C.)] is used as the plasticizer, and this is put to use in an Na.sup.+ sensor and K.sup.+ sensor. It was subsequently reported that DOS (dioctyl sebacate), which has a high lipophilic/hydrophilic distribution coefficient, is best suited for an ion-selective electrode [Anal. Chemistry 52 (4) 692-700 (1980)]. However, this concept does not hold true in a case where the ion carrier membrane is of a solid-state type. In such case, the stability between the ion carrier membrane and an electrolytic polymeric membrane takes on great importance.
In the prior art, it is known to incorporate an ion carrier substance in the ion carrier membrane used in an ion-selective electrode, in which a polyvinyl chloride resin serves as a matrix and a plasticizer serves as the solvent. Examples of the plasticizer which can be used are phthalate esters, maleates, sebacates and adipates, though flow-out of these plasticizers and the toxicity of the effluent pose problems. In particular, it is known that the amount of plasticizer outflow increases in blood or body fluids. For this reason, a plasticizer which exhibits little outflow is desirable as the membrane for an ion sensor of the type used to subject blood or body fluids to measurement.
In Japanese Patent Application Laid-Open Nos. 61-155949 and 61-194343, etc., the inventors have previously reported on an ion sensor obtained by depositing a redox layer on an electrically conductive substrate, and coating the result with an ion carrier membrane. From the standpoint of durability, it is desired that the plasticizer in these ion carrier membranes have little solubility with respect to the redox layer composition.